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The source code for a Windows Trojan capable of recording Skype calls as MP3 files has been released in a move that spells bad news for VoIP confidentiality.

Swiss software developer Ruben Unteregger published the source code for a package capable of eavesdropping on encrypted Skype VoIP conversations on his website Megapanzer.com. Unteregger, who claims he developed the software while working for ERA IT Solutions, reportedly published the code in order shed light on a dark subject and to spur the development of security defences.

An interview on his site touches on Unteregger's rationale for releasing the source code for the Skype Trojan under the GPL.

The source code of this wiretapping trojan will be published in the upcoming days. There won’t be problems about copyright, because ERA IT Solutions let me keep it. The code will be published, it will get analysed as soon as the binaries got uploaded, signature patterns will be created by antivirus companies, the malware will be detected, blocked and deleted, if it tries to infect a system.

Unteregger's blog post announcing the release outlines some of the main features of the malware.

The code is simple and straightforward. You have [to] know malware development is no rocket science and if you expect big magic you are at the wrong place. The backdoor receives instructions from the dropzone and transfers audio files. The Skype-Tap intercepts the Skype function calls, extracts and dumps audio data to files, converts it to the mp3 format and encrypts it.

The code is not 100 per cent complete. I removed the plugin system in the backdor and also the firewall bypassing system is not there anymore. I will publish both of them in separate tools later. If you don’t like this… well, I can’t help you. Thats how it is. Take it or leave it.

Symantec warns that the public availability of the code (dubbed Peskyspy) is likely to spur the development of "customised" threats.

The source code includes backdoor functionality, according to preliminary analysis by anti-virus firm F-Secure.

Leaked documents have previously suggested that Bavarian authorities commissioned a firm called DigiTask to create a similar Trojan. Law enforcement agencies in the US and Europe, most vocally those in Germany, have long complained that Skype has become a barrier to investigations and called for changes in the law that would allow them to plant Trojans on suspects PCs.

The approach involves tricking users into getting infected and hoping the any security defences they have in place don't detect the threat. Not detecting a threat as part of a gentleman's agreement between a software vendor and a law enforcement agency could risk alienating customers and would be difficult, if not impossible to conceal, for any length of time.

That's just one reason why the whole law enforcement Trojan plan has long struck us as both unworkable and plain wrong-headed. The trade craft of a serious criminal or terror suspect would have to be really sloppy to get caught. Simply using a Mac or, better, Linux would defeat any law enforcement Trojan targeted at Windoze users, for example. ®